We can't use the normal `ImportMap` because we stop resolving if looking
up a request within yields a result. However, for module polyfills, we
only need to look up the request when normal resolving fails. Hence the
dichotomy between the `import_map` and the `fallback_import_map`.
In a more modular architecture, we would model this as a stack of three
resolvers:
1. ImportMapResolver
2. DefaultResolver
3. FallbackImportMapResolver
I'm guessing this is what we'll have in the future, but this requires a
bigger refactoring than what I'm comfortable with implementing for now.
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